An air conditioning apparatus comprises a compressor, a fan, and other various devices. A motor is often used as the power source for these devices. The motor is connected to a motor driver unit (called a “driver” below) composed of a plurality of switching elements, and the motor can be rotated by a drive voltage outputted by turning the switching elements in the driver ON and OFF.
Further, sometimes the rotational speed of the motor is controlled to allow the various types of devices such as the compressor and the fan to operate in appropriate state. A motor current that is passed through the motor is often used for such motor rotational speed control. Here, as a method for detecting the motor current, for example, as disclosed in Japanese Laid-open Patent Application No. 2005-192358, there is known a technology where a shunt resistor serving as a current flows and where the motor current is detected on the basis of both end voltages of the shunt resistor.